Polymer PTC components are widely used in various electrical or electronic apparatuses to prevent important elements composing the apparatus from breaking down if an excessively large amount of current flows in the power circuit, etc. Such a component itself is well known, and commonly comprises a PTC element normally in laminar form and composed of a polymer composition of conductive fillers dispersed in a polymer, and metal electrodes, e.g. metal foil electrodes, disposed on the facing main surfaces thereof.
For example, PTC devices having PTC components as described above and a lead or leads connected thereto are used in rechargeable battery packs. A battery pack has a cathode terminal at one end, and the PTC component is electrically connected to the cathode terminal via a lead.
One of requirements that a PTC component used as described above should meet is that the resistance of the PTC component itself be small at normal times. In the PTC element used in such low-resistance PTC component, metal fillers, in particular nickel or nickel alloy fillers are used as conductive fillers to be dispersed in the polymer. Such metal fillers are easily oxidized by oxygen present in the ambient atmosphere of the PTC component, as a result of which the resistance of the PTC element increases. Such increase in resistance is not desirable in a PTC component that should essentially be low resistance.
Therefore, in a polymer PTC component using such metal fillers, the measure of forming a resin coating that covers at least an exposed part is adopted in order that the exposed part of the PTC element does not come in contact with the ambient atmosphere and thus preventing oxidation of the metal filler. Since main surfaces of the PTC element are covered with metal electrodes as described above, such exposed part is mainly a side surface portion of the PTC element (that is, a side portion of a peripheral portion which defines a thickness of the planar PTC element, and therefore a surface which connects peripheral portions of opposing main surfaces of the PTC element).
Such a resin coating is basically effective in preventing the oxidation of the metal filler, but in some cases may not be sufficient depending on the environment in which the PTC device comprising the PTC component is used. For example, if the PTC component is located in an environment around which solvent is present, the resin coating may be degraded by the solvent, as a result of which it may partly break down so that oxygen may access the PTC element. In this case, the metal filler may oxidize, as a result of which the PTC component may be likely not to function appropriately.
A PTC device is shown in International Publication No. WO 1997/06538.
Therefore, it is a problem to be solved is to provide a PTC device wherein the PTC element will function appropriately even when the PTC device is used in an environment in which a substance such as a solvent is present which may affect the PTC device.